The present invention relates generally to operations performed and equipment utilized in conjunction with subterranean wells and, in an embodiment described herein, more particularly provides equipment and methods for use in underbalanced well completions.
At times it is useful to be able to isolate a portion of a tubular string, such as a production tubing, drill pipe, liner or casing string, from the remainder of the tubular string. For example, while drilling underbalanced, it is useful to be able to periodically trip a drill string in and out of the well without killing the well. In that instance, a valve may be interconnected in a casing string, the valve being opened upon tripping in the drill string, and the valve being closed when the drill string is tripped out of the well. A valve suitable for such an application is described in U.S. Pat. No. 6,152,232, the entire disclosure of which is incorporated herein by this reference.
Other uses include running completion assemblies (including perforated or slotted liners) after drilling underbalanced, drilling overbalanced in areas of lost circulation to prevent kicks and loss of mud while tripping the drill string, and drilling in deep water where pore pressure and fracture gradient provide a narrow window for acceptable mud density and use of lower mud density is desired.
From the foregoing, it can be seen that it would be quite desirable to provide improvements in underbalanced well drilling and completions, in other operations, and in equipment utilized in these operations.